1. Field of the Invention
The present invention relates to a charging apparatus and an image forming apparatus.
2. Description of the Related Art
In electrophotographic image forming apparatuses such as copying machines, printers and facsimile units, images are formed as follows. By use of a photoreceptor drum on a surface of which a photosensitive layer containing a photoconductive substance is formed as an image carrier, the surface of the photoreceptor drum is subjected to application of charges so as to be uniformly charged therewith. Subsequently, electrostatic latent images corresponding to image information are formed by various image forming processes. The electrostatic latent images are developed by a toner-containing developer supplied from a developing unit, so as to obtain visible images which are then transferred onto a recording material such as paper. After that, the visible images are fixed on the recording material by heat and pressure given by a fixing roller so that an image is formed on a recording paper.
In the image forming apparatus as described above, a charging apparatus is used for charging the surface of the photoreceptor drum. The charging apparatus is composed of: an electrode for conducting corona discharge on the photoreceptor drum; a grid which is optionally disposed between the surface of the photoreceptor drum and the electrode, for controlling an amount of charges imparted from the electrode to the surface of the photoreceptor drum, and thus controlling a potential of the surface of the photoreceptor drum being charged; a support member for supporting the electrode and the grid. Since the grid is capable of almost exactly controlling the potential of the surface of the photoreceptor drum being charged, a charging apparatus provided with the grid is becoming predominant at present. As the grid, for example, there are used a wire grid which is prepared from stainless steel or tungsten, and a porous plate grid which is formed of a metal plate (grid substrate) of stainless steel or the like with a large number of perforations.
As the electrode for the charging apparatus, for example, there are used a wire electrode and a metal plate electrode (hereinafter referred to as “needle electrode”) having a plurality of needle portions. Among them, the needle electrode is preferably used because of its advantages such as a less number of components, a longer operating life, less generation amount of ozone, and less frequent failures by virtue of no disconnection. The needle electrode is manufactured by etching a metal plate which is mainly formed of an iron-based metal material such as stainless steel, to form a plurality of needle portions in the metal plate. The needle electrode manufactured by etching is also referred to as an etched electrode. An etched cross section of the needle electrode lacks in the smoothness. Further, a plurality of edges for conducting discharge are present at the top end of the needle portion, and the shape of the edges present at the top ends of a plurality of needle portions is not uniform. Accordingly, discharges from the respective needle portions are not uniform. As a result, it is not possible to sufficiently control the potential of the surface of the photoreceptor drum being charged, so that the potential of the charged surface of the photoreceptor drum becomes uneven.
Further, the material for the needle electrode, i.e. the iron-based metal material such as stainless steel has high durability, but involves a drawback of being easily oxidized due to water content under a high humidity circumstance and ozone generated by corona discharging during a charging operation. In addition, in a long-time use of the needle electrode, a use of the needle electrode under the high humidity circumstance, a contact of the needle electrode with ozone, etc. are inevitable. Accordingly, the needle electrode formed of a metal material such as stainless steel corrodes due to moisture in air or due to ozone etc., resulting in deterioration of durability thereof. In addition, there is lowered controlling performance for high voltage applied to the needle electrode for conducting the corona discharge from the needle portion, with the result that the potential of the discharged surface of the photoreceptor drum is uneven, leading a to-be-solved problem that the surface of the photoreceptor drum cannot be constantly charged to a desired potential in a stable manner.
Further, as in the case of the needle electrode, the wire electrode also involves a to-be-solved problem such that rust or corrosion occurs due to ozone generated by corona discharge, with the result that the potential of the charged surface of the photoreceptor drum is uneven.
In view of the foregoing problems in the charging apparatus, there has been proposed, for example, a charging apparatus including: a wire electrode extended in a shield case having one surface thereof open; and a plate grid arranged between the wire electrode and the photoreceptor drum, the plate grid which is formed by applying a nickel plated layer of about 1 μm thickness on a surface of a porous stainless steel plate and further forming thereon a gold plated layer of about 0.3 μm thickness (refer to, for example, Japanese Unexamined Patent Publication JP-A 11-40316 (1999)). In the plate grid of JP-A 11-40316, the gold plated layer is formed by way of the nickel plated layer, with the result that the gold plated layer is less easily peeled off, and the corrosion resistance and the controllability of the potential of the surface of the photoreceptor drum being charged are relatively satisfactory. However, the manufacture of the plate grid requires two plating steps, i.e., nickel plating and gold plating. This imposes drawbacks such as more complicated manufacturing step and increase of the cost. Further, in order to make this plate grid sufficiently exhibit the preferred characteristics as described above, a thickness of the gold metal layer needs to be set at 0.3 μm or more. In addition, since the plate grid is a relatively large member having a substantially the same size as the photoreceptor drum, the usage of gold is necessarily increased also because of the necessity for increasing the thickness of the plated layer. However, such a heavy usage of gold unnecessarily increases the cost of the charging apparatus and thus the cost of the image forming apparatus, resulting in loss of the general applicability of the image forming apparatus based on its relatively low price, which is one of the advantages of the image forming apparatus. Accordingly, there has been a demand for realizing, without using an expensive material such as gold, a charging apparatus having the needle electrode and the plate grid which are excellent in durability and controllability of the potential of the surface of the photoreceptor drum being charged.
Further, there has been proposed a charging apparatus including: a wire electrode; and a plate grid which is obtained by forming a gold plated layer directly on a surface of a stainless steel metal plate by an electrolytic plating method using a pulse current, as in the case of JP-A 11-40316 (refer to, for example, Japanese. Unexamined Patent Publication JP-A 2001-166569). Also in this plate grid electrode, the gold plated layer is less easily peeled off and, like the plate grid in JP-A 11-40316, the corrosion resistance is high and the controllability of the potential of the surface of the photoreceptor drum being charged are also favorable. However, also in this plate grid, a thickness of the gold metal layer needs to be set at 0.3 μm or more, it involves the same drawbacks as those in the charging apparatus of JP-A 11-40316.
Further, there has been a corona charging apparatus having a wire electrode, at least a shaft of which is formed of iron-tungsten-based amorphous metal (refer to, for example, Japanese Unexamined Patent Publication JP-A 61-98368 (1986)). There is an advantage that a use of the wire electrode of JP-A 61-98368 can decrease unevenness in charge on the surface of the photoreceptor drum. However, even such a wire electrode does not overcome the drawback of being easily oxidized due to ozone, to a sufficiently satisfactory level. Accordingly, in order to impart long-lasting durability to the wire electrode, a coating layer formed of metal or the like material must be formed on a surface of the shaft formed of iron-tungsten-based amorphous metal.
Further, in all of the charging apparatuses of JP-A 11-40136, JP-A 2001-166569, and JP-A 61-98368, in the case of forming images by use of toner which contains as an external additive silica having a surface thereof hydrophobized by trimethylsilyl group-containing-polysiloxanes (hereinafter referred to as “hydrophobic silica” unless particularly mentioned), the polysiloxanes are attached to the plate grid or the wire electrode. Due to the foregoing, the above charging apparatuses thus have drawbacks of easily causing charging defects. At present, the toner containing the hydrophobic silica is an essential constituent for speeding up image formations in an electrophotographic image forming apparatus.
On the other hand, it has also been proposed to coat the electrode with gold (refer to, for example, Japanese Unexamined Patent Publication JP-A 2004-4334). The charging apparatus of JP-A 2004-4334 includes a needle electrode on a surface of which a coating layer made of gold, platinum, copper, nickel or chromium is formed by plating. In JP-A 61-98368, a method of etching, precision pressing, or the like method is used for forming the needle electrode, but the cross section of the needle electrode obtained by the method lacks in smoothness and results in fine irregularities. Accordingly, even after applying the plating, fine irregularities on the cross section remain as they are and may disturb the balance of the corona discharge, resulting in uneven potential of the charged surface of the photoreceptor drum. Further, contaminants such as toner are easily deposited on the fine irregularities. That is, the needle electrode of JP-A 2004-4334 has a drawback that contaminants such as the toner are attached thereto during long time use and this causes the charged surface of the photoreceptor to have further uneven potential.